# deduction guides for `std::priority_queue`

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< cpp‎ | container‎ | priority queue

Defined in header `<queue>` |  |
template< class Comp, class Container > priority_queue( Comp, Container ) -> priority_queue<typename Container::value_type, Container, Comp>; | (1) | (since C++17)
template< class InputIt, class Comp = std::less</*iter-value-type*/<InputIt>>, class Container = std::vector</*iter-value-type*/<InputIt> > priority_queue( InputIt, InputIt, Comp = Comp(), Container = Container() ) -> priority_queue</*iter-value-type*/<InputIt>, Container, Comp>; | (2) | (since C++17)
template< class Comp, class Container, class Alloc > priority_queue( Comp, Container, Alloc ) -> priority_queue<typename Container::value_type, Container, Comp>; | (3) | (since C++17)
template< class InputIt, class Alloc > priority_queue( InputIt, InputIt, Alloc ) -> priority_queue</*iter-value-type*/<InputIt>, std::vector</*iter-value-type*/<InputIt>, Alloc>, std::less</*iter-value-type*/<InputIt>>>; | (4) | (since C++17)
template< class InputIt, class Comp, class Alloc > priority_queue( InputIt, InputIt, Comp, Alloc ) -> priority_queue</*iter-value-type*/<InputIt>, std::vector</*iter-value-type*/<InputIt>, Alloc>, Comp>; | (5) | (since C++17)
template< class InputIt, class Comp, class Container, class Alloc > priority_queue( InputIt, InputIt, Comp, Container, Alloc ) -> priority_queue<typename Container::value_type, Container, Comp>; | (6) | (since C++17)
template< ranges::input_range R, class Comp = std::less<ranges::range_value_t<R>> > priority_queue( std::from_range_t, R&&, Comp = Comp() ) -> priority_queue<ranges::range_value_t<R>, std::vector<ranges::range_value_t<R>>, Comp>; | (7) | (since C++23)
template< ranges::input_range R, class Comp, class Alloc > priority_queue( std::from_range_t, R&&, Comp, Alloc ) -> priority_queue<ranges::range_value_t<R>, std::vector<ranges::range_value_t<R>, Alloc>, Comp>; | (8) | (since C++23)
template< ranges::input_range R, class Alloc > priority_queue( std::from_range_t, R&&, Alloc ) -> priority_queue<ranges::range_value_t<R>, std::vector<ranges::range_value_t<R>, Alloc>>; | (9) | (since C++23)

The following deduction guides are provided for std::priority_queue:

1-6) Allow deduction from underlying container type and from an iterator range. /*iter-value-type*/<It> denotes typename std::iterator_traits<It>::value_type for any type `It`.
7-9) Allow deduction from a `std::from_range_t` tag and an input_range.

These overloads participate in overload resolution only if

- `InputIt` satisfies LegacyInputIterator,
- `Comp` does not satisfy Allocator,
- `Container` does not satisfy Allocator,
- for overloads (4,5),(since C++23) `Alloc` satisfies Allocator, and
- for overloads (3,6), std::uses_allocator_v<Container, Alloc> is true.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type `Alloc::value_type` must exist and the expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | `202202L` | (C++23) | Ranges-aware construction and insertion; overloads (7-9)

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <queue>
    #include <vector>

    int main()
    {
        const std::vector<int> v = {1, 2, 3, 4};
        std::priority_queue pq1{std::greater<int>{}, v}; // deduces std::priority_queue<
                                                         //     int, std::vector<int>,
                                                         //     std::greater<int>>
        for (; !pq1.empty(); pq1.pop())
            std::cout << pq1.top() << ' ';
        std::cout << '\n';

        std::priority_queue pq2{v.begin(), v.end()}; // deduces std::priority_queue<int>

        for (; !pq2.empty(); pq2.pop())
            std::cout << pq2.top() << ' ';
        std::cout << '\n';
    }

Output:

    1 2 3 4
    4 3 2 1

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3506 | C++17 | deduction guides from iterator and allocator were missing | added

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